高强度汽车渗碳齿轮钢的发展及应用

渗碳齿轮钢是汽车用主要结构钢之一,国内主要汽车生产厂家的高强度齿轮钢材料一直使用不同国家多个牌号的渗碳钢。随着技术和应用的发展有必要开发新品种的高强度齿轮钢。分析了高强度汽车渗碳齿轮钢在国内外的发展及应用现状,并根据我国合金材料的特点展望了其发展趋势。     

Effect of hot‐rolling conditions on the tensile properties of multiphase steels exhibiting a TRIP effect

TRIP‐assisted multiphase steels have been thoroughly studied in the cold‐rolled and annealed state. The effects of hot‐rolling conditions on these steels are much less studied even though these are of major importance for industrial practice. This study was carried out in order to understand the effect of the hot deformation of austenite on the tensile properties of TRIP‐assisted multiphase steels. Two different compositions and microstructures are investigated. The first one is a low‐carbon steel (mass content of 0.15 %) with a microstructure consisting of an intercritical ferritic matrix, bainite and retained austenite. The second one is a medium‐carbon steel (mass content of 0.4 %) that consists of bainite and retained austenite. Both steels were deformed to various strain levels below the non‐recrystallisation temperature of austenite. The medium carbon steel was deformed in the fully austenitic temperature range whereas the low‐carbon steel was deformed in the intercritical temperature range. In both cases, the prior hot deformation of austenite brings about a large enhancement of the work‐hardening capabilities. In the case of the medium‐carbon steel, this effect can be attributed to a much larger TRIP effect taking place during straining. In the case of the low‐carbon steel, the improvement of the work‐hardening behaviour was attributed to an Interaction between the martensitic transformation and the dislocations already present within the surrounding ferrite matrix.     

低碳多相钢的组织调控与力学性能

采用优化后的临界区再加热-淬火中温等温(T1、T2)热处理工艺,对具有不同前躯体组织的(0.22/0.17)C-(1.91/1.85)Mn-(1.32/0.94)Si两类热轧6 mm钢板分别进行处理,获得了具有铁素体、贝氏体、马氏体以及弥散分布于原奥氏体晶界、相界等处的残余奥氏体所构成的多相组织.利用扫描电镜、X射线衍射以及电子背散射衍射分析技术等对不同热处理阶段钢的微观组织进行了表征.结果证实,采用不同的前躯体组织设计可以很好地调控临界区再加热逆转变奥氏体的组织形貌、比例以及碳含量,进而通过后续处理来实现对钢中多相组织的调控.前躯体为马氏体的0.22C钢,经T1工艺后获得了以针状铁素体为基体的多相组织,其强塑积超过了30 GPa·%;前躯体为铁素体+马氏体的0.17C钢经T2工艺后获得了以块状铁素体为基体的多相组织,其强塑积超过了27 GPa·%.     

双面搪瓷用含钛热轧高强钢的组织和性能

简述了工业生产的一种热轧双面搪瓷专用钢的成分和生产工艺,并与一种普通的碳素结构钢进行了组织、性能对比研究。结果表明,这种搪瓷专用钢经高温搪烧后的晶粒组织仍非常细小,且基体中弥散分布着大量的Ti C、Ti_4C_2S_2等析出相。在800~890℃间进行两次高温搪烧后,其屈服强度仍可以达到342 MPa的较高水平。氢渗透试验结果表明,搪瓷专用钢的贮氢性能远优于作为对比样的普通碳素结构钢。经双面湿法搪烧后可以获得良好的搪瓷层气泡结构,并且不会出现鳞爆缺陷。     

Development of a hot‐rolled Nb‐bearing Si‐TRIP steel with excellent fatigue behaviour for automotive applications

A hot‐rolled TRIP steel with a tensile strength level above 800 MPa and an excellent fatigue behaviour has been developed with the purpose of satisfying the increasing demands of the automotive industry. Nb was added to the steel to guarantee a stable process and to make the production of the steel possible in a conventional rolling mill. The formation of red scale and the surface quality problems that arise from it have been studied. Additionally, hot torsion tests have been performed in order to validate a new route that could avoid the formation of this red scale allowing the application of the steel for exposed parts (e.g. wheel sector).     

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On industrial scale, high strength P- containing IF (Interstitial Free) steels were produced with both batch annealing and continuous annealing processes and their microstructures and properties were studied. Forming abilities of the two steels were compared with commercial Al- killed steel DC01 and extreme deep drawing IF steel DC04 produced with batch annealing. The results show that the microstructures of high strength IF steels are composed of equi- axial ferrite and their mechanical properties meet the requirement of relevant standard. The batch annealed high strength P- containing steels show poorer formability than continuous annealed ones. The test results of texture and orientation distribution function (ODF) demonstrate that density of ??- fiber increases in order of batch annealed high strength P- containing IF steel, commercial DC01, continuous annealed high strength P- containing IF steel and normal IF steel DC04.     

Modelling of austenite decomposition of hot‐rolled plain carbon steels under complex cooling conditions

The purpose of the present work is to develop a mathematical model allowing the simultaneous prediction of both transformation product portions and mean ferrite grain size from the same common principles as a result of austenite decomposition during continuous cooling of plain carbon steels. The transformation products considered specifically are polygonal ferrite and pearlite. The model is based on the classical equations of nucleation‐growth theory and also contains some empirical parameters. The chemical driving forces for nucleation and composition of elements at the phase interfaces are derived from thermodynamic analysis. Three modes of ferrite nucleation are taken into account that correspond to the nucleation on the austenite grain corners, edges and faces. The model considers the reduction of the nucleation sites due to the occupation of austenite grain boundary surface by ferrite grains. Pearlite transformation starts at the γ/α interface and suppresses further ferrite grain growth. The parameters related to ferrite reaction were determined on the basis of a series of austenite transformation kinetic curves and grain size measurements for a steel with the composition 0.084%C‐0.58%Mn‐0.02%Si obtained by dilatometric technique for cooling rates from 0.032 to 2.5 K/s. The parameters related to pearlite reaction were determined on the basis of the data for a steel with 0.66%C. After determination of the model parameters the model was applied to complex cooling conditions of the run‐out table of the hot strip mill at Voest‐Alpine Stahl Linz GmbH. Predicted ferrite grain size appeared to be 1.2 ?1.3 times smaller than the observed one. With regard to experimental data on grain growth in iron, it was suggested that the underestimation of grain size is due to additional ferrite grain growth occurring after the coiling of the steel sheet. Taking that into account provided satisfactory agreement with observed values.     

武钢高强度汽车用钢板开发与应用

汽车更轻和更安全是人们一直努力追求的目标,采用高强度钢板能为实现这个目标做出贡献。经过多年研发,武钢已开发出一系列的高强度汽车用钢板,使用这些钢板能使汽车减重并提高汽车的安全性。为满足汽车工业的需要,武钢今后将开发更高强度的汽车用钢板。     

Microstructure ‐ tensile strength correlation of salt bath nitrocarburised engineering steels

A commonly used engineering steel (En8) and a hot working die steel (H11) were salt bath nitrocarburised in the ferritic and austenitic range and studied for their microstructure and tensile properties. Compound layer thickness and hardness increased with increasing treatment temperature. There was a marginal improvement in the tensile strength after ferritic nitrocarburising due to the hardness increase in the compound layer and diffusion zone. There was marginal drop in the tensile properties of specimens treated in the austenitic range, but these values were comparable to those obtained after blank treatment at the same temperature. The tests showed that the compound layer had good cohesion with the underlying zone despite the precipitous hardness drop at their interface.     

Microstructure and properties of high strength aluminium alloys for structural applications

This article discusses the fundamental basis of high strength Al alloy design and describes the role of alloying elements, mechanical processing parameters and heat treatments toward the evolution of microstructure that controls the desired properties i.e. strength, fracture toughness, stress corrosion cracking (SCC) resistance, fatigue crack initiation and propagation resistance, and weldability in 7xxx series Al alloys. The beneficial effects of suitable micro/trace alloying elements, and deleterious effects of certain impurity elements on a variety of properties are further discussed within the present context.     

高伸长率冷轧双相钢DP980显微组织及性能

摘要：为了进一步提高冷轧双相钢DP980的强塑性,采用低C-Si-Mn-Nb-Cr成分,通过调整连续退火工艺参数,系统研究了工艺组织性能的关系,利用OM、SEM、EBSD分析了不同退火温度条件下各相的比例、尺寸、形貌、分布,同时利用力学拉伸试验手段研究了连退两相区退火温度对强塑性的影响。结果表明,通过优化调整连续退火工艺,不仅可以在冷轧铁素体和马氏体双相钢的组织上获得少量的残余奥氏体,也能细化再结晶晶粒,最终获得ReL/Rm≤0.5、高伸长率A50≥15%的冷轧DP980,提高强塑性的同时改善了成型性能。     

汽车用高强低合金钢成形性能的研究

对本钢HC260LA、HC340LA、HC420LA系列汽车用高强低合金钢进行金相组织检验、常温力学性能测试和成形极限试验,并将力学性能试验和成形试验结果导入AUTOFORM仿真软件,进行冲压仿真模拟。结果表明:本钢汽车用高强低合金钢系列产品力学性能和成形性能良好,加工硬化指数n≥0.11,塑性应变比r值约1.0,伸长率A_g≥12%,在平面应变状态下的极限应变值FLD0大于20%,满足生产汽车纵梁等结构件的要求。     

Delayed fracture of tensile strength 980 MPa grade steels for automotive applications

Three different types of tensile strength( TS) 980 MPa grade advanced high-strength steels used in automotive applications,namely,980 MS( martensite steel),980DP( dual phase) and 980QP( quenching and partitioning) steels were examined. The delayed fracture resistance of the steels was evaluated using a U-bend test,slowstrain rate test( SSRT) and a constant load tensile test. The results indicated that all the steels could pass the300h HCl solution immersion test and none of the U-bend specimens was fractured in the test. However,the steels exhibited different susceptibilities to delayed fracture under SSRT and the constant load tensile tests. 980 DP exhibited the highest resistance to delayed fracture among all the samples,while 980 MS was found to be the most susceptible to delayed fracture.     

Development of high strength low alloy TRIP‐aided steels with annealed martensite matrix

Formable high‐strength low‐alloy TRIP‐aided sheet steels with annealed martensite matrix or TRIP‐aided annealed martensitic steel were developed for automotive applications. The steels possessed a large amount of plate‐like retained austenite along annealed martensite lath boundary, the stability of which against the strain‐induced transformation was higher than that of the conventional TRIP‐aided dual‐phase steel with polygonal ferrite matrix. In a tensile strength range between 600 and 1000 MPa, the TRIP‐aided annealed martensitic steels exhibited superior large elongation and reduction of area. In addition, the steels possessed the same excellent stretch‐flangeability and bendability as TRIP‐aided bainitic steel with bainitic ferrite matrix. These properties were discussed by matrix structure, a strength ratio of second phase to matrix, retained austenite stability, internal stress in matrix and so on.     

Discharging properties of tundish slag for an improved hot‐tundish recycling process

The discharging properties of tundish slag for an improved hot‐tundish recycling process were investigated using a lab‐scale discharging experiment at 1773 K. The sticking ratio of the slag (defined as the weight ratio of the sticking slag on a Al₂O₃ crucible after discharging to the total slag at the moment of discharging) was found to decrease with increasing basicity ((%CaO)/(%SiO₂)), Fe_tO content and discharging temperature, and decreasing Al₂O₃ content. While the substitution of MgO for CaO increases the sticking ratio, Al₂O₃ and SiO₂ show nearly identical effects on it. The experimental results were discussed in terms of the physical properties of slag, viz. viscosity and wettability. Finally, both sticking ratio and erosion tendency were linked with optical basicity, and an optimum region of slag composition for an improved hot‐tundish recycling process was proposed. With the proposed slag condition, the suitability of the plant slags for an optimized process was investigated.     

Stress corrosion cracking of high‐strength steels

The threshold stress intensity of stress corrosion cracking (SCC) in the NaCl solution, K_ISCC, has been measured for five low alloy steels. The effects of yield strength, alloy elements, microstructure and grain size on K_ISCC were studied. The results showed that K_ISCC decreased exponentially with increasing yield strength, σ_ys, i.e., K_ISCC = 1.38 · 10⁶exp(‐8.26 · 10^‐3σ_ys) for 40CrMoV steel and K_ISCC = 1.42 · 10⁶exp(‐4.66 · 10^‐3σ_ys) for 30CrMnSiNi steel. For low‐alloy high‐strength steels with σ_ys = 1400 MPa, the effect of alloy elements, microstructure and grain diameter larger than 7 μm on K_ISCC was little. The threshold stress intensity of hydrogen‐induced cracking during dynamical charging for 40CrMoTi steel decreased linearly with the logarithm of the concentration of diffusible hydrogen, C₀, i.e., K_IH = 31.3‐9.1lnC₀. This equation was also applicable to SCC of a high‐strength steel in aqueous solution, and in this case, C₀ is constant. The critical hydrogen enrichment concentration, C_th, necessary for SCC of high‐strength steel in water decreased exponentially with the increase in yield strength. It was possible to deduce the relationship between K_ISCC and σ_ys, i.e., K_ISCC = Ak₁exp(‐k₂σ_ys), where A = 3RT√πρ /2(1 + ν) , k₁ and k₂ are constants, which depend upon the compositions and microstructure of the steel as well as the test conditions.     

Composition and processing of direct-quench hot rolled steels with ultrahigh strength exceeding GPa

The design of high-strength steel has long been discussed in the field of metal structural materials. To further increase the strength of common high-strength s...     

Factors influencing the strength differential of high strength steels

Room temperature tensile and compressive true stress-true strain curves of various high strength steels (quenched and tempered 4340 steel, 410 martensitic stainless steel, and H-11 steel; and aged 300-grade 18 Ni maraging steel) were analyzed to determine the effect of the various microstructures, on what has been termed the strength differential (SD),i.e., the strength level difference between the tensile and compressive flow curves. Care was taken to insure that the compressive deformation was homogeneous. Regardless of the amount of plastic deformation, the quenched and tempered steels exhibited a higher flow stress in homogeneous compressive deformation than for tensile deformation. The extent of the SD was dependent on tempering temperature. This observation is consistent with what others have observed regarding yield strength behavior of quenched and quenched-and-tempered steels. Despite the low carbon content, aged maraging steel also showed a greater resistance to homogeneous compressive deformation. Metallographic examination of the maraging steel revealed the banding that is indicative of segregation. However, homogenization had little effect on the SD despite a change in austenite grain size, reverted austenite content, and the austenite-to-martensite transformational strains shown by Goldberg to be present in segregated material.     

终轧温度对热轧高强集装箱用钢组织及性能的影响

利用金相显微镜、扫描电镜以及透射电镜等检测设备,研究了不同终轧温度对高强集装箱用钢的组织、性能及第二相粒子析出行为的影响。研究结果表明;不同终轧温度下高强集装箱用钢的组织均由铁素体和少量珠光体组成,铁素体晶粒度均为13.0级;850℃终轧与890℃终轧相比,屈服强度与抗拉强度分别提高32 MPa与26 MPa。理论计算与透射电镜观察结果表明:高强集装箱用钢强度的提高主要是因为850℃终轧时第二相粒子的形核率更高、析出数量更多、粒子间距更小,析出强化效果更加显著。